Modern wind mills, also called wind turbines or wind engines, are used to produce electricity. They are often very large structures with blades of up to and in excess of 60 meters and made from fibre-reinforced polymer structures, such as shell elements. These wind turbines are provided with control devices which may prevent an overloading of the wind turbine and the blades at wind gusts and high wind speeds. Such control devices can also be used to slowing the turbine down and bringing it to a complete halt, if the wind speed becomes too high. In addition to these devices the turbine may comprise a braking device arranged in communication with the main shaft of the wind turbine.
The control devices may be formed of pitch-controlled blades mounted such on the hub that they are able to turn about the longitudinal axis. The blades may thus be continuously adjusted to provide the lift rendering the desired power. In so-called stall-controlled wind turbines the blades are fixedly mounted on the hub and thus unable to turn about their longitudinal axis. The stall properties of the blades are used to reduce the aerodynamic lift and thus the power output.
U.S. Pat. No. 6,361,275 (Wobben) discloses a wind turbine, wherein the pitch angles of each wind turbine blade can be adjusted independently such that corrections for varying wind speeds in the swept area of the wind turbine blades can be made. The wind speed often increases with the distance to the surface of the ground and it may thus be advantageous to adjust the lift power of the blades such that the load on the blade is substantially uniform during its entire rotation. However, this solution may also be used to compensate for other types of variations in the wind speed in the rotor plane.
The wind turbine according to U.S. Pat. No. 6,361,275 is, however, encumbered by the drawback that the adjustment cannot be made particularly fast, the conventional pitch hydraulics of the blade being used for this purpose. The hydraulics is not adapted for fast and almost instantaneous movements, since a wind turbine blade, whose dead load may exceed 10 tons, has a comparatively large inertia.
As mentioned above, particularly at large wind turbines, e.g. offshore wind turbines, the wind speed may vary greatly locally in the area swept by the rotor. The rotor may have a diameter of more than 120 meters, for which reason the wind speed may vary greatly due to local wind gusts and turbulence both in height and width.
U.S. Pat. No. 4,692,095 discloses a wind turbine blade according to the preamble to claim 1.
WO 2004/099608 also discloses a wind turbine blade according to the preamble of claim 1, wherein the size of the zone with reduced lift can be varied gradually by use of active flaps.